The present invention is directed to a method and apparatus for measuring the thickness and sheet resistance of metal coatings disposed on semiconductor wafer products.
As semiconductor wafers increase in size, the costs involved in the production of these wafers also increase. Additionally, the semiconductor industry continues to demand higher yield outputs from manufactured semiconductor wafers, as well as having an ongoing demand for quickly produced, high quality, semiconductor products. As such, there is a continual need for nondestructive testing, conducted either inline during the semiconductor manufacturing process or by way of a standalone unit, to measure and monitor the thickness of metal deposition on semiconductors wafers. Additional need exists for nondestructive testing for semiconductor wafers that have undergone a chemical machining process.
It is well known that an eddy current can be used to measure a material""s thickness as well as its conductance, as illustrated in U.S. Pat. No. 4,849,69. In the ""694 Patent, a microscope is used to maintain the eddy current probe at repeatable and precise distances above a measured sample. However, this approach is slow and cumbersome for inline production monitoring of metal film thickness of wafers. For example, the focal point location of the microscope is different than the location of the detecting eddy current sensor. As such, even if the microscope can maintain a constant distance above the sample (e.g., wafer), this technique does not provide an eddy current sensor at a constant distance above the measured sample.
Another well known technique for determining the thickness of a semiconductor wafer is illustrated in U.S. Pat. No. 4,727,322. In the ""322 Patent, a predetermined value of one component is set and acts as a gate trigger. The wafer""s thickness, which is in the range of a calibration curve, can be determined by the value measure at some predetermine value.
Other typical application of an eddy current measurement is described in U.S. Pat. No. 5,552,704. In this Patent, a system is described as being capable of measuring the conductance (e.g., conductivity, resistance, or resistivity) on a sample using an eddy current probe, without the need to measure the separation between the probe and sample. However, in the ""704 system, a minimum of 25 data points are needed to generate the lift-off curve of all of the known conductance wafers. The system also generates a calibration curve by pre-selecting a curve to intersect all of the life-off curves. The unknown sample can then be measured by finding the intersecting point between the calibration curve and the unknown sample curve.
Because of the ""704 Patent utilizes a pre-selected curve to intersect the known conductance lift-off curves, it does not accurately represent the conductance as a function of conductance. The ""704 Patent""s method only provides an estimation of unknown conductance when the intersected value is plugged into the conductance function. As such, the pre-selected curve does not represent a true conductance function versus intersecting point.
It is to be further noted that although traditional systems are able to obtain accurate calculations of metal coating thicknesses; however, these systems often utilize methods that destroy the inspected sample. In these types of systems, a standard or electron microscope is utilized to measure the thickness of a wafer""s coating after a cross-section has been cut through the coating.
The present invention is capable of measuring the thickness of metal coatings disposed upon semiconductor wafer products, as well as calculating sheet resistance from a known resistivity constant.
The terms calibration sample and inspection sample will be repeatedly used throughout the specification. The calibration sample term denotes a material sample having a known thickness and resistivity. The calibration sample is utilized during a calibration session to obtain a variety of baseline measurements. The inspection sample term denotes a sample having a material layer where the thickness and sheet resistance are unknown.
The present invention preferably obtains several different thicknesses measurements from a calibration sample that cover the possible range of thicknesses of the inspection sample. Preferably, the present invention includes a single absolute eddy current probe comprised of a probe housing and a spring load. In one embodiment, the eddy current probe housing is mounted in a vertical position, perpendicular to the surface of the measuring surface (i.e., wafer surface). However, the present invention is not so limited and other configurations are possible. For example, in another embodiment, the eddy current probe is mounted in a horizontal position, parallel to the wafer surface.
The present invention utilizes an instrument, such as an eddy current personal computer (PC) card that is configured to operate with a PC having a hard drive and CPU. The PC typically will include the necessary software to support the eddy current PC card and as well as perform the necessary data collection.
During a system calibration session, a calibration sample is measured to produce a set of data values associated with the known thickness of the sample. It is to be understood that the calibration sample (i.e., a sample having a known thickness and resistivity), and the unknown thickness sample (i.e., the inspection sample) comprise identical materials. During the calibration session, the measurement frequency generated is at 10 MHz or higher.
The measuring starting point of the calibration sample is denoted by locus (0,0), which is typically a null point or reference point, and indicates the starting locus of the eddy current signal. In other words, locus (0,0) defines the starting point of data collection of the known (i.e., calibration sample) thickness metal coating on the semiconductor wafer.
At the beginning of the calibration session measuring process, the eddy current probe is placed into contact with the calibration sample. More particularly, the probe is positioned so that an eddy current sense coil contacts the top surface metal coating of the calibration sample.
The present invention utilizes a spring load inside the eddy current probe housing to ensure that the calibration sample and eddy current probe (i.e., eddy current sense coil) remain in contact during the calibration process. This spring load ensures that the eddy current signal readings of the calibration sample are obtained from an absolute fixed distance.
Data obtained from the calibration sample consists of an X voltage value and an Y voltage value for each of the series of data samples taken during the calibration session. The X voltage value represents resistance, while the Y voltage represents reactance. Accordingly, the calibration sample provides a series of voltage data values, (X, Y), which are each associated with a known thickness.
For example, after the calibration session (i.e., a measurement of the known thickness sample) is performed, a series of data points will have been measured (e.g., data points A, B, C, D and E). Each data point (A, B, C, D and E) has an associated (X, Y) thickness value denoting their respective thickness (e.g., 500, 1000, 1500, 1700, 2000 angstroms). That is, data point A corresponds to a thickness of 500 angstroms, while data point E has a corresponding thickness of 2000 angstroms.
Since each data point has a voltage value (i.e., X voltage value representing resistance, Y voltage value representing reactance), as well as an associated thickness value (e.g., 500 angstroms, 1000, angstroms, etc.), one of ordinary skill will recognize that each data point will have associated voltage values as well as identifiable thicknesses. As such, each of the identified thicknesses (e.g., 500, 1000, 1500, 1700, 2000 angstroms) may be associated with a particular voltage value (X, Y).
Once the data points have been measured and the required voltage values have been obtained, additional calculations may be performed to determine an equation that represents the graph created by the generated data. In particular, the (X, Y) voltage values associated with each of the data points A, B, C, D, and E, may be used to find the best fit equation. As such, each of the five data points (A-E) may be plotted based on their respective voltage values. As utilized by the present invention, an appropriate equation may be either a first order or second order equation, such as:
Y=aX2+bX+C
The generated, best fit, equation represents the natural thickness intercepting curve and will be utilized to determine an intersecting point between two separate graphs (i.e., the natural thickness intercepting curve and the equation representing the inspection sample).
Once the calibration session has been performed, measurements of an inspection sample may then be performed. During the inspection sample measuring process, several data points, having associated (X, Y) values, are collected along a locus of the inspection sample (i.e., the unknown thickness sample). It is to be realized that in contrast to the calibration session, it is not necessary for the eddy current probe (i.e., the eddy current sense coil) to contact the inspection sample.
At the beginning of the inspection sample measuring process, the first data values, (X, Y), are collected by an eddy current probe that is 75 microns above the surface of the inspection sample (i.e., the unknown thickness sample). As the measuring process progresses, a motion controller retracts the eddy current probe along a vertical axis (Z axis) so that the eddy current sense coil is drawn away from the inspection sample. Thus, the distance between the eddy current probe and the surface of the inspection sample is increased during the measuring process. The retraction of the eddy current probe may be controlled by a computer.
During the inspection sample measuring process, a series of voltage data values (X, Y) will be collected. Typically, a total of 15 voltage data values are obtained during the inspection sample measuring process. The first voltage data value (X, Y) will be obtained when the eddy current sense coil is 75 microns above the surface of the inspection sample. The remaining 14 data values are obtained from distances that are incrementally further away from the inspection sample""s surface.
In particular, each of the remaining voltage data values are obtained from a distance of 25 microns from the location where the previous data value was obtained. For example, the first voltage data value is obtained from a distance of 75 microns from the inspection surface; the second voltage data value is obtained from a distance of 100 microns; the third voltage data value is obtained from a distance of 125 microns, etc. As such, it is to be understood that the eddy current probe does not contact the sampling surface.
Similarly to the method described during the calibration session, once 15 voltage data point (X,Y) values have been collected from the inspection sample additional calculations may be performed to determine an equation that represents the graph created by the generated data. In particular, the (X,Y) voltage values associated with each of the 15 data points may be used to generate a best fit equation. In other words, each of the fifteen data points may be plotted based on their respective voltage values. As utilized by the present invention, an appropriate equation may be an equation, such as:
Y=mX+B
The best fit equation generated from the inspection sample data will be referred to as the inspection sample curve.
Accordingly, at this point, two separate equations will have been determined. Specifically, the natural thickness intercepting curve and the inspection sample curve; the natural thickness interception curve having been determined from data obtained during the calibration session and the inspection sample curve determined from data obtained from the inspection sample measuring process.
The determination of an intersection point (point P) of two equations is the next calculation that may be performed. More particularly, the intersecting point of the natural thickness intercepting curve and the inspection sample curve may be calculated. The intersection point calculation may be performed by utilizing the well known mathematical method of determining an intersection point of two curves by equating the equations that represent the curves.
The result of this calculation reveals the intersecting point, P, of the natural thickness intercepting curve and the inspection sample curve. The terms, P(X,Y) and P will be used, interchangeably, to denote the intersection point of these two equations.
After the intersection point P is found, a vector impedance (Z) for each of the identified (X,Y) voltage values is then calculated. More particularly, a vector impedance is calculated for each of the data points (A-E), as well as for the intersection point P. The variable Z will be used to represent the vector impedance for each of the identified voltage values (X,Y). For clarity, the vector impedance notation will be as follows: Z(A) represents the data point A(X,Y); Z(B) represents the data point B(X,Y), Z(P) represents intersection point P(X,Y), etc. To calculate Z for each of the data points (A-E), and intersection point P, the following equation may be used:
z (X=(X2+Y2)
At this point of processing, the vector impedance Z(A-E), as well as Z(P) are known. By identifying where, among the known thickness points, that the intersection point P is located, the thickness of the inspection sample may be calculated. More specifically, by identifying where Z(P) is located with respect to Z(A-E), the thickness of the inspection sample at a particular point may be identified.
For example, if the intersection point P is located between data points A and B (i.e., Z(A) less than Z(P) less than Z(B)), then a simple interpolation between A and B will determine the thickness of intersection point P (i.e., the thickness of the inspection sample.
In situations where a scanning measurement is performed over the diameter of the inspection sample (i.e., a scan across the diameter of a semiconductor wafer), a display may be utilized to represent a cross-section of the thickness profile of the inspection sample. An example of a contour map with elevation levels that can be generated is shown in FIG. 5.